First, that Donald is not the master negotiator / business genius he claims, and is not open to taking advice from smarter people.
Second, misdirection. It may be something to draw attention - the whole 'who needs batteries for their car company?' thing - or it might be a decoy offer, never intended to be seriously contemplated, but will serve as an easy-for-the-dumbs-to-understand future-date hand-wavey explanation for why negotiations failed. (This seems more palatable if you already believe the current US administration is compromised.)